This invention relates generally to filters for the removal of solids from a fluid by directing the fluid through a filter medium, or septum, under pressure. More specifically, the invention pertains to a control method and apparatus for automatically backwashing the filter medium in response to a pressure difference across the same in order to make it free of accumulated solids, or cake. The fluid to be filtered may be liquid or gas, and either the liquid may be valuable, or the solids, or both. The filter according to the invention finds use in nuclear or steam power plants and a variety of other industrial and public service installations.
Filters are available in various forms depending in part upon the kind of septum in use, whether it is backwashed or not, and, if it is, how. Take, for example, one of the most familiar filters having a tubular filter medium. The fluid to be filtered is directed from the outside into the inside of the filter medium, and the septum is automatically backwashed to cleanse it of cake each time the cake accumulates to a prescribed amount.
Conventionally, the amount of the cake on the filter medium has been determined solely on the basis of a fluid pressure difference across the septum. This practice is objectionable because the pressure difference varies not only with the cake amount but also with the temperature and flow rate of the fluid being filtered. If the fluid changes in temperature or flow rate during the progress of filtration, as is frequently the case, it has become impossible to know the cake amount by relying on the pressure difference only. Then, unless no measure is taken to make the pressure difference independent of variations in fluid temperature and flow rate, the filter medium has been backwashed either prematurely or belatedly.
The applicant is aware of some conventional solutions to this problem. They are all unsatisfactory, however, as will be later discussed in more detail with reference to the drawings attached hereto.